How To Reduce Quantity ? [shopping Cart]

protected void Buy_Click(object sender, EventArgs e)

{SqlConnection conn = new SqlConnection();

SqlCommand cmd = new SqlCommand();

try

{String connStr = ConfigurationManager.ConnectionStrings["project_DataFile"].ConnectionString;

conn.ConnectionString = connStr;

conn.Open();

cmd.Connection = conn;ArrayList cart = (ArrayList)Session["Cart"];

{String oldSel = "SELECT Qty_warehouse FROM Producttable";

Int32 sel = 0;Int32.TryParse(oldSel, out sel);

string num = cart.Count.ToString();Int32 numQty = 0;

Int32.TryParse(num, out numQty);int nQty_warehouse = 0;

nQty_warehouse = sel - numQty;String sql = "UPDATE Producttable" + " SET Qty_warehouse =" + "'"+ nQty_warehouse +"'";

cmd.CommandText = sql;}

}catch (Exception ex)

{

Response.Write(ex.ToString());

}

finally

{if (conn.State.Equals(ConnectionState.Open))

{

conn.Close();

Response.Write("end.aspx");

}

}

 

from this code, i want to reduce product's quantity in database.

when I click buy button,it reduce old quantity and new quantity (quantity from cart when I want to buy it) in auto. 

please advise me.